1. Field of the Invention
The invention pertains to a low-pass filter specially designed to be made in the form of an integrated circuit.
A very commonly used second-order low-pass filter cell is the so-called Sallen and Key cell shown in FIG. 1. It has two resistors R1, R2, two capacitors C1, C2 and a unit gain amplifier with high input impedance and low output impedance.
2. Description of the Prior Art
The values of the resistors and capacitors regulate the shape of the frequency response curve of this cell. A response curve shape that is often sought to be obtained is the one corresponding to a second-order transfer function called the Butterworth transfer function, namely a transfer function with a damping coefficient equal to the converse of the square root of 2. For this special transfer function corresponds to the flattest possible response curve in the pass-band (there being no surge voltage near the cut-off frequency).
It can be shown that, for the Sallen and Key cell of FIG. 1, this special response curve is obtained on condition that the value of the capacitor C1 is twice that of the capacitor C2 and on condition that the resistors R1 and R2 have the same value: EQU C1=2C2 and R1=R2.
If the resistors R1 and R2 are made by a diffusion of low-concentrated P-type impurities in a monocrystalline silicon substrate, a low space factor is got for the cell. However, this is got at the cost of a certain distortion of the signal owing to the non-linearity of these resistors.
If the resistors R1 and R2 are made of polycrystalline silicon, the non-linearity and distortion disappear but then the resistors as well as the capacitors take up considerably more space. In one example, it was observed that, to obtain the same response curve, it was necessary in one case (using polycrystalline silicon) to have resistors that were 40 times bulkier and capacitors that were 6 times bulkier than in the other case (with P.sup.-).
Other arrangements of filters with distributed constants have been proposed. These filters are designed to fulfill the same overall low-pass filtering function, but do so using circuit elements with distributed resistances and capacitances instead of the circuit of FIG. 1 which has resistances and capacitances that are clearly distinct from one another. These arrangements using distributed constants have been proposed in order to improve the attenuation beyond the cut-off frequency. For their attention may be exponential and, therefore, better than the attenuation of a true second-order filter such as the Sallen and Key cell.
The element with distributed constants may typically be a polycrystalline silicon line (displaying a certain degree of resistance) formed by etching a resistive layer of polycrystalline silicon, insulated by a dielectrical thin film of a conducting layer of polycrystalline silicon or any other conducting layer constituting a plate of a capacitor, the other plate of which is a relative line.
But the low-pass filtering cells with distributed constants proposed until now are not of very high quality as regards the overall space factor of the cell for a given response curve shape (especially for a given cut-off frequency).